Thousands of people staged demonstrations in cities across America Sunday and into Monday -- resulting in more than a dozen arrests in New York City and Los Angeles -- as they voiced their disappointment and anger at the decision by a Sanford, Fla. jury's to acquit George Zimmerman in the fatal shooting of 17-year-old Trayvon Martin.

The 29-year-old neighborhood watch volunteer, on Saturday, was found not guilty of second-degree murder and manslaughter in the shooting of Martin in February 2012.

Most of the protests were peaceful, but some of the most dramatic scenes occurred in Los Angeles, where protesters clashed with police Sunday night and Monday morning, with one group of protesters blocking a major freeway.

Police in Los Angeles said they arrested six people on Monday, mostly for failure to disperse.

The Los Angeles Times reported that a splinter group from a larger protest walked down an on-ramp to the Interstate 10 freeway in the Mid-City section of Los Angeles and stood in the eastbound lanes, closing portions of the traffic artery for approximately 25 minutes, which prompted a Los Angeles Police Department citywide tactical alert.

The Times also quoted an LAPD spokesman who said that at least one arrest was made after rocks and D-cell batteries were thrown at officers at the corner of Washington Boulevard and 10th Avenue. Police also fired non-lethal rounds at the demonstrators. No injuries were immediately reported.

The city's mayor, Eric Garcetti, took to Twitter to ask city residents to "Exercise [the] 1st Amendment and practice peace in City of Angels tonight."

Early Monday, the Times reported that more than 100 officers in riot gear converged on around 80 protesters in front of the CNN building on Sunset Boulevard in Hollywood and made arrests. The protesters had marched to the CNN building from the intersection of Hollywood Boulevard and Highland Avenue.

In New York City, more than a thousand people marched into Times Square on Sunday night, zigzagging through Manhattan's streets to avoid police lines. Sign-carrying marchers thronged the busy intersection, chanting "Justice for! Trayvon Martin!" as they made their way from Union Square, blocking traffic for more than an hour before moving on.

Some tempered their anger, saying they didn't contest the jury's decision based on the legal issues involved.

But "while the verdict may be legal, a system that doesn't take into account what happened is a broken legal system," said Jennifer Lue, 24, an Asian-American resident of Harlem.

The New York Police Department said it arrested at least a dozen in the Times Square protest.

At a march and rally in downtown Chicago attended by about 200 people, some said the verdict was symbolic of lingering racism in the United States. Seventy-three-year-old Maya Miller said the case reminded her of the 1955 slaying of Emmitt Till, a 14-year-old from Chicago who was murdered by a group of white men while visiting Mississippi. Till's killing galvanized the civil rights movement.

"Fifty-eight years and nothing's changed," Miller said, pausing to join a chant for "Justice for Trayvon, not one more."

Protesters also gathered in Atlanta, Miami, Milwaukee, Philadelphia, and Washington, D.C., along with a host of other cities.

In Miami, more than 200 people gathered for a vigil. "You can't justify murder," read one poster. Another read "Don't worry about more riots. Worry about more Zimmermans."

Carol Reitner, 76, of Miami, said she heard about the vigil through an announcement at her church Sunday morning. "I was really devastated. It's really hard to believe that someone can take the life of someone else and walk out of court free," she said.

Fears of mass and violent protests proved unfounded Sunday in South Florida the face of a highly-visible police presence.

"I haven’t seen any evidence of problems yet, and hopefully there won’t be any," Ed Shohat, a Miami-Dade’s Community Relations Board member, told The Miami Herald. "We do not believe (violence) will happen. Frankly, Miami is a … more mature community than … 25, 30 years ago when we had violent reactions to criminal court verdicts."

In Philadelphia, about 700 protesters marched from LOVE Park to the Liberty Bell, alternating between chanting Trayvon Martin's name and "No justice, no peace!"

"We hope this will begin a movement to end discrimination against young black men," said Johnathan Cooper, one of the protest's organizers. "And also to empower black people and get them involved in the system."

In Atlanta, a crowd of about 75 protesters chanted and carried signs near Centennial Olympic Park.

"I came out today because a great deal of injustice has been done and I'm very disappointed at our justice system; I'm just disappointed in America," Tabatha Holley, 19, of Atlanta said.

Civil rights leaders, including the Revs. Jesse Jackson and Al Sharpton, urged peace in the wake of the verdict. Jackson said the legal system "failed justice," but violence isn't the answer.

But not all the protesters heeded those calls immediately after the verdict.

In Oakland, Calif., during protests that began late Saturday night some angry demonstrators broke windows, burned U.S. flags and started street fires. Some marchers also vandalized a police squad car and used spray paint to scrawl anti-police graffiti on roads and Alameda County's Davidson courthouse.

The Oakland demonstration followed a raucous but largely peaceful rally in San Francisco. Police say officers escorted demonstrators as they marched on the city's Mission District. The group was dispersed by 10 p.m.

Meanwhile, Mark O'Mara, who defended Zimmerman at trial, suggested his client’s safety was at risk. "There still is a fringe element that wants revenge," O'Mara said. "They won't listen to a verdict of not guilty."

O'Mara told ABC News that Zimmerman will be entitled to get his Kel Tec 9 pistol back, and his client would likely arm himself again.

"[There's] even more reason now, isn't there? There are a lot of people out there who actually hate him, though they shouldn't," he said.

O'Mara said Zimmerman wears a protective vest when he goes out in public, but he did not wear it in court.